Corona station for the preliminary processing of a strip material

ABSTRACT

The present invention relates to a corona station for the preliminary processing of a strip material. The corona station includes at least one electrode (18) connected to a high-voltage power supply through connection means, a housing containing at least partially the electrode (18) and the connection means, and at least one cylindrical counter-electrode (17), wherein the strip of material (16) does not rest on the whole surface of said counter-electrode (17).

CROSS-REFERENCES TO RELATED APPLICATIONS

Not applicable.

Statement Regarding Federally Sponsored Research or Development

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to a corona station for pretreating on both sidesa material web.

TECHNICAL FIELD

EP 0 426 880 has disclosed a corona station for treating plastic webmaterials which have a pattern created in surface portions thereof whichis achieved as a direct result of a charge treatment. Therefore thematerial web is brought into contact with a portion of a circumferentialsurface of a counter-electrode. The circumferential surface of thecounter-electrode has been engraved with a pattern for producing aselectively patterned, surface treated web.

Furthermore, a non-generic device for removing dust particles onmaterial webs has been disclosed by EP 0 524 415 A2, an has tworod-shaped emission electrodes which run parallel to one another and areconnected to an opposite potential, and between which the material webruns through. In the process, the dirt particles are charged, with theresult that the latter can easily be detached from the material web andsucked off. Although this device has emission electrodes, it cannot beused for pretreating material webs. Even if this device were to be usedthe following disadvantages would occur. In this device, there is a needupstream and downstream of the emission electrodes for guide rolls whichare rotatably mounted and must be coordinated with one another in termsof their circumferential speed in such a way that a certain web tensionis maintained as the material web is being guided through the device. Onthe one hand, this is required in order not to damage the material webby making contact with the emission electrodes, and on the other hand inorder to ensure a uniform air gap on the topside and bottom siderelative to the emission electrodes so that adequate surface treatmentis rendered possible. Moreover, this device also has a design which iscomplicated in terms of apparatus. The disadvantages arising therefromwith regard to the space requirement, mounting, maintenance etc., areobvious. Moreover, the failure rate by comparison with a single coronastation is twice as high.

SUMMARY OF THE INVENTION

It is therefore the object of the invention to create a corona stationwhich renders it possible to treat completely the surface of thematerial web on both sides in a simple and cost-effective way, withoutdamaging the surface of the material web.

This object is achieved by an air nozzle provided at least upstream ofthe counter-electrode, seen in the conveying direction of the materialweb so that the material web does not bear with its whole surfaceagainst the counter-electrode.

Owing to the fact that the air nozzle is provided at least upstream tothe counter-electrode it is possible by at least partially blowing inair between the material web and the counter-electrode that the materialweb does not bear with its whole surface against the counter-electrode.It is therefore possible to form a more or less completely constructedair gap between the counter-electrode and the material web to betreated. Consequently, both the surface of the material web pointedtowards electrodes and the surface of the material web pointed towardsthe counter-electrode are completely surface-treated, since an air gapor a quasi air gap is formed on both surfaces. Due to blowing in airbetween the material web and the circumferential surface of thecounter-electrode a plurality of air gaps in the depressions act likeone quasi air gap. The corona treatment, rendered possible by thedepressions, of the surface is emitted onto the linear or punctiformcontacts of the material web on the counter-electrode, so that afterleaving the counter-electrode the surface, pointing towards thecounter-electrode, of the material web is essentially wholly surface-treated by a spill-over effect. Therefore, by contrast with priorattempts not to carry out any treatment on the counter-electrode, thisdevelopment has chosen an avenue in the opposite direction, inaccordance with which it is now possible to treat completely a materialweb on both sides with the aid of a corona station. It is ensuredthereby that the material web does not bear its whole surface againstthe counter-electrode, but touches the surface covering thecounter-electrode only very slightly in a linear or punctiform fashion.The construction of a grid is a feature of the invention in which thesurface of the depressions is constructed into a multiple of theelevations, with the result that, on the one hand, an adequate bearingsurface is created for the surface bearing against the counter-electrodeand, on the other hand, the grid does not give the surface a structure.

According to a refinement of the invention, a further electrode isprovided immediately after the material web lifts off thecounter-electrode. The spill-over effect will be additionallystrengthened.

According to a further refinement of the invention, it is provided thatthe grid extends perpendicular to the transport direction of thematerial web. The result is to enable the material web on thecounter-electrode to be uniformly unwound and/or deflected without itbeing possible for the material web to be warped or deviate from thetrue course with respect to the conveying direction. Moreover, it ispossible to counteract the formation of folds during deflection.

According to a further refinement of the invention, it is provided thatthe depression are arranged in a plurality of different grid patterns,or are distributed over the circumferential surface into partiallyidentical or different grid patterns. As a result, it is possible totreat only specific surface sections, for example. Consequently, aspecific pattern can be rendered possible on the basis of the printingwhich follows, for example, since without corona treatment thecorresponding sections cannot accept coating materials.

The depression are introduced without cutting into the circumferentialsurface by means of a knurling device. Furthermore, it is advantageouslypossible to provide for it to be possible by laser machining or sparkerosion, or by other alternative machining operations such as, forexample, by chemical removal or the application of a coating, to achievea grid-shaped or structured arrangement of depressions, or else also anirregular arrangement of depressions or a surface roughness whichpermits the formation of an at least partial air gap between thecounter-electrode and that surface of the material web which is to betreated.

According to a further refinement of the invention, it is provided thata suction device is provided on the housing of the corona station orseparately therefrom for the corona-treated surface on thecounter-electrode. As a result, the ozone gas can be sucked off anddisposed of or preprocessed in accordance with the safety rules.

The invention renders it possible for a device, known to date from theprior art, for one-sided web treatment now to be used and alsoretrofitted for two-sided material web treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained with the aid of a preferredexemplary embodiment. In the drawing:

FIG. 1 shows a diagrammatic representation of corona station accordingto the invention,

FIG. 2a shows diagrammatic view of a counter-electrode, configuredaccording to the invention, with a knurled pattern which runs transverseto the transport direction,

FIG. 2b shows a diagrammatic view of the counter-electrode, configuredaccording to the invention, with a knurled pattern which is of rhombicdesign,

FIG. 3 shows a greatly enlarged section of mutually adjoining knurledpatterns in FIGS. 2a and b of a counter-electrode,

FIG. 4 shows an enlarged diagrammatic cross-sectional representation ofthe knurling configured transverse to the transport direction,

FIG. 5 shows a diagrammatically enlarged cross-sectional representationof the rhombic knurled pattern, and

FIG. 6 shows a diagrammatically enlarged top view of the knurled patternin accordance with FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a corona station 15 for pretreating a material web 16 whichis subsequently printed, coated or the like. The material web 16 can beany substrate which can be conveyed continuously or intermittently inmachines or rolls. For example, such substrates can be homogeneousmaterials, likewise foam plastics or mesh materials which can have bothconducting and nonconducting properties. For example, partiallyconductive materials can be assigned to the conductive materials ormaterial webs. The geometry of the material web is mostly such that thethickness of the material web is smaller than the width. For example, inthe case of pretreating a plastic film the thickness can vary from 0.01to 5 mm, and the width from, for example, 30 to 10,000 mm. It goeswithout saying that the adaptation in width and thickness of thematerial webs to be treated can also be adapted to specific cases.

The corona station 15 has electrodes 18 which are assigned to acounter-electrode 17. The counter-electrode 17 is preferably designed asa non-driven roll. The material web 16 wraps around the counter-rolls17, for example by 90° to 180°.

The voltage for the electrodes 18 is generated in a generator, it beingpossible to vary the appropriate power as a function of the treatment.The corona station 15 represented in FIG. 1 corresponds essentially tothe corona station, known from the prior art, for one-sides pretreatmentof material webs 16.

The corona station 15 according to the invention differs, however, inthe configuration of the counter-electrode 17, which is adapted as afunction of the material to be pretreated. By contrast with thecounter-electrode known from the prior art, the counter-electrode 17according to the invention does not have a smooth surface, butdepressions dog, which are represented by dashes.

This exemplary embodiment with a counter-electrode 17 constructed fromconducting material permits non-conducting material webs to bepretreated. In this refinement, the electrodes 18 are supplied with highvoltage, whereas the counter-electrode 17 has an earth potential. It canalso be notched. Owing to the configuration of the depressions 26 on thecircumferential surface of the counter-electrode 17, it is possible forthe material web 16 not to bear with its whole surface against thecounter-electrode 17, and for a sort of air gap 26 to be formed suchthat the corona discharge takes place in the air gap 26. In addition tothe customary air gap 26′, formed between electrodes 18 and the materialweb 16, of a corona station 15 for one-sided web treatment, it isthereby possible to achieve the formation of a second quasi air gap 26between the counter-electrode 17 and the material web 16, in order topermit double-sided pretreatment of the material web 16 by coronadischarges.

In the treatment of a conductive material web 16, it is provided thathigh voltage is present both at the electrode 18 and at thecounter-electrode 17, the earth potential being present at theconductive material web 16. The counter-electrode 17 has acircumferential surface which is constructed as a dielectric and inwhich depressions 25 are also provided. The counter-roll 17 isadvantageously mounted in an insulated fashion. A conductive materialweb 16, for example aluminum foil, metal foils, metallized foils of thelike can therefore be pretreated on two sides by means of coronadischarge.

Provided advantageously downstream and/or to the side of thecounter-electrode 17 seen in the withdrawal direction of the materialweb 16 is a suction device 51 by means of which the ozone formed by thecorona discharge can be sucked off.

For the purpose of forming the quasi air gap 26 upstream of thecounter-electrode 17 seen in the conveying direction of the material web16 it is provided an air nozzle 52 which supports the formation of theair gap 26. The air nozzle 52 can extend over the entire width of theroll, or act only partially. Furthermore, the slot width of the airnozzle 52 can have sections differing in length and/or width.

It can advantageously be provided, furthermore, that an electrode 18 isarranged immediately after the material web 16 lifts off from thecounter-electrode 17, in order to strengthen a spill-over effect on thesurface pointing towards the counter-electrode 17, or to carry out anadditional corona discharge.

In the following FIGS. 2 to 6, exemplary embodiments of depressions 25and their cooperation with the material web 16, which does not bear withits whole surface against the counter-electrode 17, are discussed inmore detail.

A counter-electrode 17 designed as a roll is represented in FIG. 2a. Ithas depressions 32 which are arranged sagitally relative to thegeometrical center axis 24. These depressions 32 form a knurling 22which is constructed parallel to the geometrical rotation oflongitudinal axis 24. This longitudinal correlation is provided at rightangles to the conveying direction for transporting the material web 16

An alternative embodiment of the depressions 36 is represented in FIG.2b. These form a rhombic knurling 23 which, by analogy with thedepressions 32, are likewise formed sagitally relative to thegeometrical center axis 24.

The concrete configuration of the depressions 25, 32, 36 can have amultiplicity of variations, it being necessary to set certain minimumpreconditions, as will be further explained below.

The counter-electrode 17 can have only one knurling 22 or 23, forexample, on its circumferential surface. However, it is also possible toprovide an arbitrary arrangement of the knurlings 22, 23, if appropriatealso only partially, on the counter electrode 17, it being possible forthem to be adapted as a function of the respective application.Moreover, further types of depressions can be provided in combination.

It is also possible to provide the depression 25 only in sections bothin the radial and in the axial directions, so that, for example, thepretreatment can apply a type of pattern to the material web 16, as aresult of which partial surfaces of the material web 16 aresurface-treated and other partial surfaces remain untreated.

It is also possible to provide that individual depression are ofrelatively large configuration, so that, on the basis of the largeunoccupied space between the material web 16 and the counter-electrode17, they act as insulation, as a result of which no pretreatment isperformed in this region. Consequently, single-sided pretreatment of thematerial web 16 can be performed in sections on the surface facing theelectrodes 18. This effect can also be provided by using insulatingmaterial in the counter-electrode 17.

In accordance with the present state of knowledge, the linear knurling22 in accordance with FIG. 2a has very similar or identical propertiesto the knurling 23 in accordance with FIG. 2b. However, it isconceivable that further modifications of the knurling 22 or 23 withregard to the configuration of the surface and the size of thedepressions 32, 36 and also with regard to the width of the depressions32, 36 and to the size and shape of the sections 33 situatedtherebetween can deliver further positive effects which render possiblea partial and/or complete surface treatment of the material web 16.

Represented by way of example if FIG. 3 is an enlargement of atransition from the knurling 22 to the knurling 23. The transition doesnot impair the pretreatment of the surface in this region when such acombination is provided on a counter-electrode.

FIG. 4 shows a greatly enlarged view of a cross section of the knurling22. The arrow 29 shows the direction of rotation with respect to thegeometrical longitudinal axis 24. This can also be directed oppositelyin accordance with the arrow 31. This can be advantageous, for example,whenever a roll consumed during operation in the direction 39 is rotatedthrough 180° so that the direction of rotation is then in accordancewith the arrow 31.

The knurling 22 has oblong depressions 32 which run parallel to thegeometrical axis 24 and can have a U-shaped or rectangular cross sectionor the like. The sections 33 constructed between the depression 32 andwhich form this circumferential surface can advantageously lie on aradius of circle and be curved. Alternatively, it can be provided thatthese sections 33 are of rectilinear construction, with the result thatwhen the material web 16 is wrapped around the circumferential surfaceof the counter-electrode 17 a small cavity forms between the outercorner points 34 of the sections 33 so as to reduce the bearing surfaceof the material web 16 relative to the circumferential surface of thecounter-electrode. The configuration of the width, and the spacing ofthe oblong depressions 32 are such that the material web 16 does not saginto the oblong depression 32, or does so only negligibly. The depth ofthe depression 32 is adapted to the width of the depression 32, with theresult that it can be ensured that the material web cannot come intocontact with the base of the depression. The corner points 34 canadvantageously be rounded formations, with the result that noimpressions are left behind on the material web.

The configurations of the points of contact and/or lines of contactbetween the material web 16 and the counter-electrode 17 areadvantageously mutually coordinated. On the one hand, the aim is for thesurfaces of contact and/or points of contact to be so small as toachieve by the spill-over effect of the pretreated surface that thesurfaces of contact are likewise pretreated downstream of thecounter-electrode 17. On the other hand, the aim is for thecounter-electrode 17, which is preferably not driven, to be draggedalong by the material web 16 so that damage to the surface of thematerial web 16 can be avoided. Were a complete air gap 26 to beproduced, the counter-electrode 17 would stand still, the result ofwhich would be that, in the case of a reducing air gap 26, the materialweb 16 would briefly strike the counter-electrode 17 and surface damagewould result owing to the increased friction.

Moreover, the knurling 22 can be of wave-shaped or U-shapedconstruction. Moreover, it can be advantageous, if appropriate, for thespacing of the depressions 32 to be formed nonuniformly or irregularlywhen seen radially. This can, for example, occur because the oblongdepressions 32 and/or the sections 33 can be of wider or else narrowerconstruction.

The circumferential surface of the counter-electrode 17 can be ofmetallic construction in the case of pretreating a non-conductivematerial web. Aluminum or an aluminum alloy or the like canadvantageously be used. Further materials are likewise possible. Thesurface of the counter-electrode 17 can be finished after theintroduction of the knurling 22, with the result that, for example, noburrs of sharp edges remain standing after the introduction of thedepressions 25. This can be provided, for example, by applying a layerof silicone or the like. The circumferential surface of thecounter-electrode 17 can furthermore, be finished with an anodized oxidelayer or the like, in order moreover to permit a lengthening of theservice life.

The production of the depression can, for example be rendered possiblein the case of the knurling 22 by means of customary knurlingtechniques. Moreover, depressions deviating geometrically from thisknurling 22 can be introduced without cutting or by spark erosion bymeans of laser technology, water jet cutting or etching or otherabrading processes. In the last case, in particular, an irregulararrangement can be constructed.

In the case of the surface treatment of a conductive material web 16,the circumferential surface of the counter-electrode 17 is designed witha dielectric. A production method for introducing the depressions 32, 36into the corresponding dielectric is thereby to be selected and adapted.

The non-conducive materials of a material web such as, for example,paper and plastics can likewise be treated with the aid of acounter-electrode having a dielectric.

A diagrammatically enlarged cross section of the knurling 23 isrepresented in FIG. 5. This rhombic knurling 23 is advantageouslydesigned such that no preferred direction is provided. Consequently, thematerial web 16 can be deflected on the counter-electrode 17 withoutdeviation in the direction of transport. It can also be provided that,for example, knurlings having preferred directions directed towards oneanother can respectively be provided on the outer edge regions of thecounter-electrode 17, as a result of which it is possible to force thematerial web 16 into a corresponding direction of transport. The rhombicdepressions 36 have corner points 37, 38 which are less far removed fromone another that the distance between the corner Points 39 and 41. Sincethe tensions of the material web 16 runs perpendicular to thegeometrical longitudinal axis 24, the span is therefore shorter andthere is less risk that the material web 16 will sink in and thus thedepth of the depression 36 will be reduced. Nevertheless, a sufficientlylarge air gap can be formed. Furthermore, not only are the depressionsof the type of depressions 36 present here, but so are the furrows 42and 43, which intersect at the four corner points 37, 38, 39, 41 at aconstant angle. To this extent, FIG. 5 shows only a diagrammaticsection, that is to say only the cut section and not, for example, alsothe perspectively running ridge lines from peak to peak.

The furrows 42, 43 run at the same angle with respect to thecircumferential direction and are essentially situated in a bisectingfashion relative to the geometrical longitudinal axis 24. The knurling23 therefore has a symmetrical design, with the result that the materialweb 16 has no tendency to run to the left or right. Should such atendency be desired, it can be produced by consciously producedasymmetries in the grid pattern. Further knurlings, which are similar tothat represented by way of example in FIGS. 2a, 2 b and FIGS. 4 and 5,respectively, can likewise be used.

Because of the formation of a quasi air gap 26 between thecounter-electrode 17 and the material web 16, ozone occurs in this airgap 26 through the corona discharge; it is set off via a suction device(not represented in more detail). This suction device is designed by ananalogy with the suction device at electrodes 18, and treated inaccordance with the safety rules, for example, via an ozone catalyst,with the result that the air/ozone mixture sucked off can be output intothe environment after filtering.

During operation of the corona station 15, the power output by theelectrodes 16, 17 is increased from, for example, 600 W for one-sidedpretreatment of the material web 16 to approximately 800 W for two-sidedpretreatment of the material web 16. However, this power can be set in afashion specific to the application, the power to be output essentiallybeing governed by the width of the foil and by the foil speed. Theresult of the pretreatment is that the surface tension of, for example,32 ON/mm² can be increased to a value of approximately 56 ON/mm².

Thus, by means of the invention, corona stations already in use can beretrofitted in a simple and cost-effective way for two-sidedpretreatment of the material web, this being done by exchanging thecounter-electrode 17 and adapting the power of the generator.

There is a specific numerical relationship between the contact surfacesof the points of contact, the blowing in of air and the depression onthe circumferential surface of the counter-electrode 17. On the onehand, it is necessary for the circumferential surface of thecounter-electrode 17 to define a minimum supporting surface so thatthere is no damage to or impairment of the material web 16 owing topeaks or depressions or the like, and so that, on the other hand, an airgap of sufficient size can be achieve in order to render thepretreatment possible. Moreover, to treat the web over its whole surfacethe pretreatment must occupy a surface of the material web 16 such thata spill-over effect can be provided despite the contact surfaces on thecounter-electrode 17, with the result that the regions of the materialweb 16 which adjoin the counter-electrode 17 can likewise be broken upand thus pretreated on the surface, if appropriate after leaving thecounter-electrode 17. This degree of spill-over is also dependent, interalia, on the power of the electrode 18 and of the counter electrode 17.

The design of the counter-electrode 17 requires at least a coordinationof the size and shape of the depression 32, 36 with the depth of thedepression 32, 36 and also with the sections 33 situated therebetween.The web speed is also to be taken into account. In this case of speedsof the material web 16 below approximately 200 m/min., the material webis scarcely raised from the counter-electrode 17, with the result thatthe grid is to be of correspondingly smaller construction, that is tosay the lines or points of contact are more closely interspaced. Theeffect occurring from the web speed is dependent in turn on the angle ofwrap of the material web 16 on the counter-electrode 17 and is to betaken into account, the smaller the angle of wrap, the higher thepossible web speed, and the further from one another the lines or pointsof contact can be spaced. The web speed is also to be adapted so that itremains possible to control the lateral course. It is advantageous tocoordinate the design of the knurling with the sensitive materials, lowweb speeds and high angle of wrap, in order to create acounter-electrode 17 which can be used universally.

What is claimed is:
 1. A corona station for pretreating a material web,having at least one electrode, which can be connected to a high-voltagesupply via connecting means, a housing which at least partiallyaccommodates the electrode and the connecting means, and having at leastone counter-electrode of cylindrical construction, the counter-electrodehas depressions on its circumferential surface, the depressions arearranged at least in subregions in the form of a regular grid,characterized in that an air nozzle is provided at least upstream of thecounter-electrode, seen in the conveying direction of the material weband the material web does not bear with its whole surface against thecounter-electrode.
 2. The corona station according to claim 1,characterized in that a further electrode is provided immediately afterthe material web lifts off the counter-electrode.
 3. The corona stationaccording to claim 1, characterized in that transverse to the transportdirection the grid has at least no substantial preferred direction. 4.The corona station according to claim 1, characterized in that thedepressions are distributed in a plurality of different grid patterns orin partially identical or different grid patterns over thecircumferential surface of a counter-electrode.
 5. The corona stationaccording to claim 1, characterized in that the depressions arepreferably introduced or applied in the circumferential surface of thecounter-electrode without cutting.
 6. The corona station according toclaim 5, characterized in that the depressions are impressed into thecircumferential surface with the aid of a knurling tool.
 7. The coronastation according to claim 1, characterized in that a suction device isprovided on the housing, or in a fashion indirectly connected thereto,for the corona-treated underside of the material web.
 8. The coronastation according to claim 1, characterized in that for pretreating anon-conductive material web the depressions are provided directly in aconducting circumferential surface of the counter-electrode.
 9. Thecorona station according to claim 1, characterized in that forpretreating a conductive of partially conductive material web thecounter-electrode is provided with high voltage, and thecounter-electrode has a circumferential surface which is provided with adielectric and in which the depressions are provided.
 10. The coronastation according to claim 9, characterized in that the conductive orpartially conductive material web has an earth potential.
 11. The coronastation according to claim 9, characterized in that thecounter-electrode is rotatably mounted in an insulated fashion.